1. Field of the Invention
The present invention relates generally to heat exchange units for cooling a product, and more particularly to freezers including heat exchange units which are adapted to be utilized in gravity and orientation independent circumstances, particularly such as in space installations.
2. Description of the Prior Art
Spacecraft and space technology has advanced to the point where lengthy missions are feasible and permanent in-orbit stations are being developed for deployment in space. With the time astronauts spend in space extending to longer periods, there is a greater desire to provide foods other than the typical space meals which are typically maintained at medium temperatures. For example, there is a desire to provide frozen or cold food, such as ice cream, soft drinks or other products, for consumption. In the zero-gravity or near vacuum environment of space, it is desirable to accomplish such in-orbit refrigeration of goods with minimal power consumption and in as compact a manner as possible. This is particularly important in the consideration of long term space voyages and permanent stations, in which the difficulties of transporting frozen or refrigerated food to the inhabitants will be extremely high.
Conventional heat exchange units are generally ineffective in the zero-gravity or near vacuum environment of space. This is because known heat exchange units which perform well at normal atmospheric pressure use pressurized refrigerants which, in space applications, may leak into a confined space and have potential to explode. Additionally, in a zero-gravity environment, there is a problem with separating liquid from gas. In particular, conventional refrigerators and freezers do not work in zero gravity because they rely on gravity to separate the liquid refrigerant from gas.
In space, the only mechanism for rejecting heat is by radiation. The equation for the rate of heat rejection per unit area (heat flux) is: EQU q'=.epsilon..sigma.(T.sub.s.sup.4 -T.sub.surr.sup.4) (1)
where .epsilon..sigma.=emissivity properties of the heat exchanger materials.
T.sub.s =heat exchanger surface temperature, i.e., temperature heat is rejected at. PA1 T.sub.surr =temperature of the surroundings that the heat is being rejected into.
Generally, space vehicles require minimum weight, size, vibration, noise, maintenance and power consumption for its freezer system which will store frozen food for the crew. Although the surrounding temperature is very cold in space, there is no air or other matter to transfer rejected heat into. Consequently, rejecting heat by radiation is a slow process that requires huge surface areas, such as surface areas in the range of 6000 square feet for the space station, to dissipate excess heat. Enormous heat exchange surfaces thus must be attached to a space vehicle to reject the excess heat into space.
What is needed therefore is a gravity and orientation independent space vehicle freezer including a heat exchange device which is safe, energy efficient, space efficient and lightweight.